1. Field of the Invention
The present invention relates to a fuel/air premixer used in a premixed, prevaporized-type combustor of a gas turbine which uses liquid fuel, and more particularly to a fuel/air premixer for a gas turbine combustor having at least one airblast atomizer nozzle comprising a liquid film-forming body disposed at an inlet portion of a premixing tube.
2. Description of the Prior Art
The nitrogen oxide NOx (NO and NO2) that is discharged from various combustion devices is not only harmful to the human body, but is also a cause of acid rain and the greenhouse effect, and as a result has become subject to official emission controls in industrialized nations. Gas turbines are no exception to these controls, and NOx emission standards are provided for gas turbines on national and local levels for industrial use, and on an international level for aircraft use. These emission standards appear likely to be strengthened in the future. Meanwhile, gas turbines are being operated at increasingly high operating temperatures and pressures in order to improve fuel economy, thereby promoting NOx formation. As a result, demands are being made for the practical application of low NOx combustion technology which is able to suppress the formation of NOx effectively.
As a whole, gas turbine combustors perform excess air lean premixed combustion. A feature of this combustion method is that a lean pre-mixture with a high degree of homogeneity, which is formed by mixing fuel with excess air prior to combustion, is burned. Here, “lean” refers to a state in which, according to a minimum air amount required for complete combustion of the fuel as a reference, the air amount is sufficiently large. Depending on the gas turbine type and so on, approximately double the minimum air amount is used, and hence excess air lean premixed combustion is an extremely useful method of suppressing NOx emissions. The formation rate of NOx increases exponentially in relation to the combustion gas temperature, and hence when homogeneity is low, NOx increases in the pockets having higher fuel concentrations than average far outweigh NOx reductions in the pockets having lower fuel concentrations. As the homogeneity decreases, this excess increases. Premixing is a method of increasing the homogeneity of the air/fuel mixture.
Lean premixed combustion type combustors have come into wide use, mainly in natural gas large-scale power generation gas turbines. In response, high expectations have been placed on the practical application of lean premixed combustion to liquid fuel gas turbines and aircraft gas turbines, but this is still in the development stage. The technical aspect to this is that it is far more difficult to form a pre-mixture with a high degree of homogeneity when using liquid fuel than when using gaseous fuel.
When liquid fuel is used, first the fuel is atomized, and then the generated particles are dispersed spatially by an air stream. Vaporization of the fuel particles progresses in the dispersion process, and once the fuel vapor has undergone a process of diffusion into the air, a pre-mixture is formed. Hence in the case of liquid fuel, this process is referred to specifically as lean premixed prevaporized combustion. When the temperature and pressure of the air are high, a chemical reaction may occur in the aforementioned process, possibly leading to auto-ignition. If, as a result of this auto-ignition, a flame is formed within the premixing tube and held within the interior of the tube, the premixing tube, fuel atomizer nozzle, and so on are damaged by burning. Kerosene and jet fuel contain components which decompose at comparatively low temperatures, and hence auto-ignite at lower temperatures than natural gas, which has methane as its main component. Auto-ignition does not occur immediately after the fuel is injected into the air stream, but after a certain delay. This delay shortens dramatically as the temperature and pressure rise, and enters the order of one millisecond under the inlet temperature/pressure conditions of the latest high pressure ratio gas turbine combustors.
Fuel atomization must be advanced if the injected fuel is to substantially complete vaporization within a short time. Further, the fuel particles must be dispersed over the entire cross section of the premixing tube as quickly as possible to achieve an even fuel concentration over the cross section of the premixing tube. When fuel particle dispersal is insufficient, the fuel concentration distribution remains uneven over the cross section of the premixing tube outlet even if the fuel particles are completely vaporized. It is particularly difficult to avoid this unevenness when the diameter of the premixing tube is large. Forming an air stream which spreads in the radial direction of the premixing tube is effective in the dispersal of the fuel particles. Forming a swirl within the premixing tube is also effective in transporting the fuel particles in a radial direction, and is of course also highly effective in advancing the vaporization of the fuel particles and the turbulent diffusion and mixing of the fuel vapor. However, a problem which arises when forming a swirl within the premixing tube is that regions with a low velocity are typically formed in the vicinity of the central axis such that when the swirl is strong, a back flow is formed. This increases the likelihood of so-called backfiring, in which a flame runs up through these regions within the premixing tube from the combustion chamber.
Known conventional gas turbine fuel/air premixers for use with liquid fuel include a premixer in which fuel is atomized at an inlet portion of a premixing tube taking the form of a venturi tube, and then mixed with air that is introduced into the venturi tube (for example, Japanese Unexamined Patent Application 2000-304260), and a premixer in which fuel is injected from a hole formed in the wall surface of the contracted portion of a venturi tube, whereupon the fuel is atomized by an air stream therein. FIG. 6 shows a representative aspect of the fuel/air premixer for a small gas turbine disclosed in Japanese Unexamined Patent Application 2000-304260. Fuel is atomized by a pressure swirl nozzle 69 upstream of an inlet 66a to a premixing tube 16. The atomized fuel particles are dispersed into an air stream 63 flowing into the premixing tube 16, whereupon a mixture 64 of the fuel particles and air passes through a contracted portion 66b of the premixing tube 16 and flows into a combustion chamber 65 while being reduced in speed at an enlarged portion 66c of the premixing tube 16. In this example, the premixing tube 16 widens substantially linearly downstream of the contracted portion 66b. 
In a fuel/air premixer such as that described above, fuel is atomized at or upstream of the contracted portion in order to facilitate dispersion of the fuel particles into the air stream. The dispersed fuel particles are carried downstream on the air stream while being vaporized, whereupon the fuel vapor mixes with the air to form a pre-mixture. If the enlarged portion expands excessively downstream of the contracted portion, the flow peels away from the wall surface, forming a back flow region, and hence the angle of expansion must be suppressed to no more than several degrees. If, in a premixing tube taking the form of a venturi tube, the air stream is caused to swirl in order to promote dispersion of the fuel particles and mixing of the fuel vapor and air, a back flow region is formed on the central axis of the enlarged portion, increasing the likelihood of backfiring. Hence a venturi tube form cannot be applied to a premixer with a large passage cross section. This problem can be solved by bundling together a large number of prevaporizing tubes with small passage cross sections, but this solution leads to further problems such as complication of the fuel supply system, weight increases, and so on.
Fuel/air premixers for gas turbine combustors in which an air swirler is disposed at the inlet portion to the premixing tube such that the air is caused to swirl, thereby promoting mixture with the fuel, are used widely in gaseous fuel gas turbine combustors (for example, Japanese Unexamined Patent Application H9-119639). These premixers may be applied to liquid gas turbine combustors simply by replacing the fuel nozzle with a liquid fuel nozzle (for example, Japanese Unexamined Patent Application H5-87340). FIG. 7 shows a representative example thereof, in which an air swirler 74 is disposed at an inlet 73 to the premixing tube 16, a central body 77 is disposed on the central axis of the premixing tube 16, and fuel is injected from fuel injection holes 78 on the surface of the central body 77. The central body 77 extends to the vicinity of the outlet of the premixing tube 16. As noted above, this form has the advantage of promoting dispersion and vaporization of the fuel particles, and diffusion and mixing of the fuel vapor, by means of a swirl, but is disadvantaged in that a low velocity region is formed in the central portion of the premixing tube 16. Since fuel also exists in this part, backfiring is likely to occur.
In this example, to solve the problems described above, the central body is provided on the central axis such that the cross-section of the pre-mixture passage takes an annular form, thereby decreasing the likelihood of a back flow while still applying a swirl. The problem with this type of fuel/air premixer for a gas turbine combustor is that a flame is formed at the outlet of the premixing tube, and hence the tip end portion of the central body is heated excessively by the flame and radiation from the flame. If the tip end of the central body is positioned upstream of the premixing tube outlet in an effort to suppress such excessive heat, the end of the back flow region, which had been positioned downstream of the premixing tube outlet, moves within the premixing tube, and hence the vicinity of the premixing tube outlet may be heated excessively. Moreover, the very existence of the central body wastes space and increases weight, and since the central body is supported by vanes of the air swirler attached to the inlet portion of the premixing tube, thus forming a so-called cantilever structure, there is a danger of the central body falling off due to combustion vibration or the like. Note that a form in which the air swirler at the inlet portion of the annular passage is constituted by coaxial inner and outer air swirlers, and back flows are prevented by having the two air swirlers rotate in opposite directions, is disclosed in Japanese Unexamined Patent Application H5-87340, for example.
Hence, when fuel from the tip end of a liquid film-forming body is atomized by an air stream and mixed with air in a fuel/air premixer for a gas turbine combustor, the air stream flow must be used to prevent back flows inside the premixing tube and reductions in the velocity of the mixture in a central portion of the premixing tube, and to take measures against backfiring caused by abnormal reductions in the air velocity and so on. These problems are to be solved using air stream swirling means which increase the velocity of the air stream passing through the inside of the liquid film-forming body and form a flow which spreads outward in the radial direction, thereby improving the atomization performance and mixing performance of the fuel, and thus forming a favorable mixture.